1. Field of the Invention
This invention is concerned with a method for preparing single crystal binary metal oxides having the formula ABO.sub.3, wherein A is an alkali or alkaline earth metal and B is either Ti, Nb, Ta or mixtures thereof. More particularly, the instant invention relates to the preparation of single crystal binary metal oxides with are grown noncongruently and isothermally in the presence of a reactive atmosphere.
2. Description of Related Art
It has been known that certain combinations of binary metal oxides have significantly useful ferroelectric properties. Ferroelectric properties occur when crystalline materials exhibit a permanent, spontaneous, electric polarization by means of an electric field. One such compound, barium titanate, is unique in that it is a perovskite-type compound, in that it exists in several forms, ie., tetragonal, hexagonal, cubic, etc. depending upon temperature. These crystalline modifications are important since they enable compounds such as barium titanate to also exhibit piezoelectric properties.
Piezoelectricity is the phenomenon whereby crystalline substances generate electrical charges when subjected to mechanical deformation. This property leads to applications in transducers, ultrasonic equipment, etc. The high dielectric constant and temperature coefficient of materials such as barium titanate, also lead to their use in the construction of small capacitors in temperature sensitive devices.
Compounds such as barium titanate also exhibit photorefractive properties which suggest a high potential for use in optical computing and image processing. There are a large number of other applications for which compounds such as BaTiO.sub.3 would be useful. These uses include bulk and surface wave acoustic devices, bulk modulator, modulators and switches for thin-film integrated optical circuits, sensitive thermal detectors and frequency doublers in the visible and microwave spectral regions. However, while compounds such as barium titanate are very promising for a wide variety of applications, the commercial availability of these compounds is limited. Moreover, the supply which is available is relatively impure.
High optical quality, single crystal electro-optic materials with controlled impurity background and dopant content are critical to optical data and other non-linear applications of importance to integrated circuits. Binary metal oxides of the titanate, niobate, and tantalate class, although exhibiting electro-optic properties, have not found use in the aforementioned electro-optical applications as it is difficult to grow crystals of these oxides of the requisite purity and size, e.g., greater than one centimeter in diameter.
V. Belruss, et al discuss another method of growing oxides in an article entitled "Top-Seeded Solution Growth of Oxide Crystals from Non-Stoichiometric Melts", Mat. Res. Bull., Vol. 6, pp. 899-906 (1971). More specifically, Belruss, et al claim that this technique would allow "one to avoid crystallization of the hexagonal phase of BaTiO.sub.3 without adding strontium or calcium, and avoiding the thermal strains inherent in crystals grown by the Czochralski or float zone techniques". However, when crystals are noncongruently grown as shown above, crystal size is limited by diffusion dynamics. More specifically, at the growth interface, the composition is mismatched and the solvent acts as a diffusion barrier. Thus, mass transport for constant crystal composition is diffusion limited, and the growth of large crystals therefore requires long time periods. During those periods, the risk of occluding the solvent in the growing solid exists, and thus the probability of maintaining uniform quality in a moderately sized crystal, ie., approximately 1 cm on an edge, is low. The solvent also serves as a mass transport medium for cation and anion impurities.
D. E. Rase et al, however, disclose in "Phase Equilibria in the System BaO-TiO.sub.2 ", J. Am. Ceram. Soc., 38, 102 (1955), that it would be possible with proper seeding and temperature control, to grow single crystals of BaTiO.sub.3 under equilibrium crystallization conditions from a mixture containing an excess of TiO.sub.2 (e.g., 55 mole %) by slowly cooling it from about 1600.degree. to 1300.degree. C. Rase, et al concluded that while such a method would have the advantage of lower temperature, thus avoiding contamination, it would also have a significant disadvantage. By allowing growth via the above method, such growth would be in a two-phase region over a range of temperatures rather than at a single temperature. The two phases consist of the solid (with the desired structure) and the melt. The composition of the two phases are unequal, hence noncongruence. The composition mismatch continues to increase curing the growth of the crystal which slows the growth rate.
The main drawback of conventional growth methods stems from the nonrigid exclusion of H.sub.2 O in the vapor phase and OH.sup.- (c) in the condensed phase. BaTiO.sub.3 crystals are conventionally noncongruently grown from a mixture of BaO and TiO.sub.2 wherein excess TiO.sub.2 is used as the flux. Growth proceeds by slow cooling of the melt from a temperature of 1390.degree. C. in the presence of air or oxygen. Water vapor which is present in the atmosphere surrounding the melt from which the crystal is grown, as well as in the outgas of the apparatus in which crystal growth is conducted, invariably leads to contamination of the BaTiO.sub.3 crystal by hydroxyl ion (OH.sup.-) impurity, even at the elevated temperatures employed for crystal growth. Water vapor adds on readily to the oxide (O.sup.--) melts to form the hydroxide (OH.sup.-) impurity as illustrated in the following equation: EQU O.sup.-- (c)+H.sub.2 O.fwdarw.2OH.sup.- (c), (1)
where (c) refers to the condensed phase of the oxide, which at the temperatures (1300.degree.-1400.degree. C.) employed for the solid-state growth of the crystal, is the melt phase. In the heated crystal, the reverse reaction is active and provides a mechanism for the formation of an anion vacancy, O : EQU 2OH.sup.- (c).fwdarw.O.sup.-- (c)+ O +H.sub.2 O. (2)
The progressive increase in the vacancy density leads to the production of color centers, .uparw. and .uparw..dwnarw. , and is responsible also for an abnormal oxygen-dissociation pressure: ##STR1## The use of hydrogen (H.sub.2) in the reduction of oxides accomplishes a similar degradation as follows: EQU O.sup.-- (c)+ O +1/2H.sub.2 .fwdarw.OH.sup.- (c)+ .uparw. . (4)
Note that although H.sub.2 is not provided in Eqs. (1) to (3) the same reducing agent, .uparw. , the solvated electron, is produced as in Eq. (4).
Water vapor in the air and from the outgas of the apparatus enhances the corrosive action of the oxide (BaTiO.sub.3) melt on the crucible (platinum). This mandates an operating temperature below 1400.degree. C.
The prior art procedure for growing single crystal binary metal oxides, employed oxygen or air to combat the tendency of the oxide melt to dissociate thereby producing a crystal which is oxide deficient. The water vapor present in the atmosphere and outgas of the apparatus used for the growing of the crystals also raises the corrosive action of the binary metal oxide whereby it reacts with the platinum walls of the crucible. This reaction results in the formation of hexagonal, solid Ba.sub.3 Pt.sub.2 O.sub.7 and limits the reaction temperature to that of below 1400.degree. C. A temperature below 1400.degree. C. is below the melting point of BaTiO.sub.3. At a temperature below 1400.degree. C., one can realize only congruent growth.
A method of eliminating water vapor from the environment in which binary metal oxide crystals are grown and of removing the presence of any OH.sup.- impurities formed thereby, is therefore needed.